1. Field of the Invention
This invention relates to acoustic feedback control, sometimes called an anti-Larsen device, provided in the telephone extensions of a digital telephone system. The device of this invention is more particularly designed to be installed in the extensions provided with a loud speaker and a microphone, in which the coders used for the analog/digital conversion of the voice signals are of the syllabic compression type.
2. Prior Art
At the present time, the use of telephone extensions provided with a loud speaker and a microphone is becoming more and more extensive, such extensions having several advantages. The use of such an extension enables the subscriber to do something else, for example, to take notes of something. Also, it is possible to arrange collective telephone calls since several persons present in a room provided with such an extension can participate to a same telephone call.
However, the use of such extensions involves some problems. The unavoidable acoustical coupling existing between the loud speaker and the microphone can initiate oscillations on the loop comprised of the microphone/loud speaker channels involved in the connection, the gain of said loop being able to exceed one. This effect is known as the Larsen effect and appears as unpleasant singing which is to be cancelled.
It is obvious that the relative positions of the microphones and loud speakers should be selected to reduce, as much as possible, the effect of the acoustical coupling, but all the effects cannot be cancelled in this way.
Many devices are known in the prior art to cancel the Larsen effect experienced in the analog type telephone systems. In this type of system, since the transmission of analog signals on a line always involves losses, it is necessary to provide amplifiers at the emitting end and at the receiving end and the Larsen effect can be cancelled by adjusting the gain of these amplifiers by hand or automatically.
This solution is not generally applicable to the digital systems since, in the digital systems, the damage caused to the pulses during on line transmission does not involve any weakening of the reconstructed analog signal amplitude and, consequently, it is not necessary to provide amplifiers which could be used to adjust the gain of the loop.
Various means of the prior art have been used to solve this problem. One can refer to two solutions known today and used in a system in which a connection between extensions provided with a microphone and a loud speaker, is made by two different transmission channels connecting the microphones and loud speakers of the extensions used in the call. The telephone extensions being adapted to the transmission of voice signals in the form of digital signals, each extension is provided with an analog/digital coder at the microphone output and with a digital/analog decoder at the loud speaker input.
At each end of the connection, the signal issued from the coder output represents the voice signal transmitted on a transmission channel and the signal applied to the decoder input represents the voice signal received from the second transmission channel and thus represents the voice signal applied to the input of this channel. Two measuring circuits are provided at each end, the first one being connected to the coder output and supplying, according to the digital information it receives, an output voltage representing the level of the voice signal on the corresponding channel, the second one being connected to the decoder input and supplying, according to the digital information it receives, an output voltage representing the level of the voice signal on the corresponding channel. A comparator receives these two output voltages and the comparator output signal indicates which channel input is provided with the voice signal with the highest level.
To cancel the Larsen, i.e. acoustic feedback, effect, only the use of this channel is effectively required in the call since the two connected subscribers do not generally talk at the same time.
Therefore, in one of the solutions described in the prior art, a switch is installed at each end on the channel issued from the microphone. This switch, in rest position, operates to disconnect the microphone/coder assembly from the channel and a rest signal is provided on this channel. When the comparator output signal indicates that the signal applied to the input of a channel shows a level exceeding the level of the signal applied to the input of the second channel, the corresponding channel switch is closed and the connection is made through this channel. Since, on this time, the second channel is "open", the Larsen effect does not appear. The position of the switches will change as the call proceeds according to the output signal continuously generated by the comparator.
Such a solution has a disadvantage due to the fact that a channel which should be normally open can be closed if a spurious noise appears, at the corresponding end, strong enough to cause the comparator to close said channel and, consequently, to open the second one. In this case, the call is momentarily disturbed.
In the second solution of the prior art, said switches are removed and the comparator output signal acts on a control logic circuit to modify the sensitivity of the modulators and demodulators of a same extension by modifying the magnitude of the quantizing step.
This solution is better due to the fact that the channels are never open; therefore, the above indicated disadvantage is overcome.
However, both solutions show a same disadvantage. In implementation, a number of additional circuits are required to perform the level comparison. These additional circuits are relatively complicated since each on implies the use of a pulse group analysis circuit including a pulse generator, a logic circuit to supply output pulses having a duration depending on the configuration of the digital pulses on the channel and, therefore, on the level of the signal on the channel, and an integrator to obtain, from this pulse, an analog voltage indicating the level of the signal on the channel. The time constants of the integrators being able to vary and not being absolutely equal in all the measuring circuits, errors can appear in determining the amplitude.